Open MRI magnet with uniform imaging volume

ABSTRACT

An open magnetic resonance imaging (MRI) magnet having first and second spaced-apart superconductive coil assemblies each including a toroidal-shaped coil housing containing a superconductive main coil. A generally annular-shaped resistive coil is associated with each coil assembly, being generally coaxially aligned with the associated coil assembly and being spaced radially inward and radially apart from the associated coil assembly&#39;s superconductive main coil. The resistive coils overcome the gross magnetic field distortions in the imaging volume of the superconductive main coils (created by the open space between the magnet&#39;s superconductive coil assemblies) to produce a magnetic field of high uniformity within the imaging volume.

BACKGROUND OF THE INVENTION

The present invention relates generally to a superconductive magnet(such as, but not limited to, a helium-cooled and/or cryocooler-cooledsuperconductive magnet) used to generate a high magnetic field as partof a magnetic resonance imaging (MRI) system, and more particularly tosuch a magnet having an open design and having a uniform (i.e.,homogeneous) magnetic field within its imaging volume.

MRI systems employing superconductive or other type magnets are used invarious fields such as medical diagnostics. Known superconductivemagnets include liquid-helium cooled and cryocooler-cooledsuperconductive magnets. Typically, for a helium-cooled magnet, thesuperconductive coil assembly includes a superconductive main coil whichis at least partially immersed in liquid helium contained in a heliumdewar which is surrounded by a dual thermal shield which is surroundedby a torroidal-shaped vacuum enclosure having a bore and a longitudinalaxis. In a conventional cryocooler-cooled magnet, the superconductivemain coil is surrounded by a thermal shield which is surrounded by atorroidal-shaped vacuum enclosure having a bore and a longitudinal axis,and the cryocooler coldhead is externally mounted to the vacuumenclosure with the coldhead's first stage in thermal contact with thethermal shield and with the coldhead's second stage in thermal contactwith the superconductive main coil. Nb-Ti superconductive coilstypically operate at a temperature of generally 4 Kelvin, and Nb-Snsuperconductive coils typically operate at a temperature of generally 10Kelvin.

A superconductive coil assembly of a conventional MRI system includesthree pulsed (i.e., not time-constant) resistive gradient coilsincluding a Z-axis coil which is coaxially aligned with the longitudinalaxis and which carries a pulsed electric current which may be in adirection opposite to the current direction of the superconductive maincoils. Such gradient coils are all located outside the vacuum enclosure(i.e., coil housing) in the bore. The superconductive coil assembly alsoincludes several resistive radio-frequency coils all located outside thevacuum enclosure (i.e., coil housing) in the bore.

Known superconductive magnet designs include closed magnets and openmagnets. Closed magnets typically have a single, tubular-shapedsuperconductive coil assembly having a bore and a longitudinal axis. Thesuperconductive coil assembly includes several radially-aligned andlongitudinally spaced-apart superconductive main coils each carrying alarge, identical electric current in the same direction. Thesuperconductive main coils are thus designed to create a magnetic fieldof high uniformity within a spherical imaging volume centered within themagnet's bore where the object to be imaged is placed. Although themagnet is so designed to have a highly uniform magnetic field within theimaging volume, manufacturing tolerances in the magnet and magneticfield disturbances caused by the environment at the field site of themagnet usually require that the magnet be corrected at the field sitefor such minor irregularities in the magnetic field. Typically, themagnet is shimmed at the field site by using pieces of iron, or, forNb-Ti superconductive magnets cooled by liquid helium, by using numerousNb-Ti superconductive correction coils. The correction coils are placedwithin the superconductive coil assembly typically radially near andradially inward of the main coils. Each correction coil carries adifferent, but low, electric current in any required direction includinga direction opposite to the direction of the electric current carried inthe main coils. It is also known to shim a closed magnet by usingnumerous resistive DC shim coils all located outside the vacuumenclosure (i.e., coil housing) in the bore. The resistive DC shim coilseach produce time-constant magnetic fields and may include a shim coilcoaxially aligned with the longitudinal axis and carrying an electriccurrent in a direction opposite to the current direction of thesuperconductive main coils to correct a harmonic of symmetricalinhomogeneity in the magnetic field within the imaging volume caused bymanufacturing tolerances and/or site disturbances.

Open magnets typically employ two spaced-apart superconductive coilassemblies with the space between the assemblies allowing for access bymedical personnel for surgery or other medical procedures during MRIimaging. The patient may be positioned in that space or also in the boreof the toroidal-shaped coil assemblies. The open space helps the patientovercome any feelings of claustrophobia that may be experienced in aclosed magnet design. The literature is largely silent on howsuperconductive open magnets can be made to have a magnetic field ofhigh uniformity within the imaging volume when the creation of the openspace between the superconductive coil assemblies grossly distorts themagnetic field creating a magnetic field of low uniformity within theimaging volume.

What is needed is an open MRI magnet designed to have a highly uniformmagnetic field within its imaging volume despite the gross magneticfield distortions created by the open space between the magnet'ssuperconductive coil assemblies.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an open superconductive MRImagnet designed to have a uniform magnetic field within its imagingvolume.

The open MRI magnet of the invention includes a first superconductivecoil assembly including a generally toroidal-shaped first coil housingand a generally annular-shaped first superconductive main coil. Thefirst coil housing surrounds a first bore and has a generallylongitudinal first axis. The first main coil is generally coaxiallyaligned with the first axis, disposed within the first coil housing, andcarries a first main electric current in a first direction. The open MRImagnet also includes a generally annular-shaped first resistive coilgenerally coaxially aligned with the first axis, spaced radially inwardand radially apart from the first superconductive main coil, andcarrying an electric current in a direction opposite to the firstdirection. The open MRI magnet additionally includes a secondsuperconductive coil assembly including a generally toroidal-shapedsecond coil housing and a generally annular-shaped secondsuperconductive main coil. The second coil housing is longitudinallyspaced apart from the first coil housing to create a cylindrical-shapedopen space therebetween, surrounds a second bore, and has a generallylongitudinal second axis which is generally coaxially aligned with thefirst axis. The second main coil is generally coaxially aligned with thesecond axis, disposed within the second coil housing, and carries asecond main electric current in the first direction. The open MRI magnetfurther includes a generally annular-shaped second resistive coilgenerally coaxially aligned with the second axis, spaced radially inwardand radially apart from the second superconductive main coil, andcarrying an electric current in the opposite direction, wherein thefirst and second resistive coils together correct within an imagingvolume for magnetic field inhomogeneities caused by the presence of theopen space, the imaging volume being located in the open space near thefirst and second axes. In a preferred embodiment, the secondsuperconductive coil assembly is a generally mirror image of the firstsuperconductive coil assembly, and the second resistive coil is agenerally mirror image of the first resistive coil.

Several benefits and advantages are derived from the invention. WithApplicant's open MRI magnet design, the resistive coils are chosen bymagnetic field analysis to overcome the gross magnetic field distortionswithin the imaging volume of the main coils (created by the open spacebetween the magnet's superconductive coil assemblies) to produce amagnetic field of high uniformity within the imaging volume. Applicant'shighly uniform magnetic field permits high quality MRI imaging.Applicant's open magnet design overcomes any claustrophobia feelings ofpatients. Applicant's design of an open magnet with a highly uniformmagnetic field gives access to the patient by medical personnel forsurgery or other medical procedures during high-quality MRI imaging.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate two preferred embodiments of thepresent invention wherein:

FIG. 1 is a perspective view of a first preferred embodiment of the openMRI magnet of the invention having internal resistive coils;

FIG. 2 is a schematic cross-sectional side-elevational view of the MRImagnet of FIG. 1 with a magnet floor mount added;

FIG. 3 is a view, as in FIG. 2, but of a second preferred embodiment ofthe open MRI magnet of the invention having external resistive coils;and

FIG. 4 is a schematic cross-sectional view of the MRI magnet of FIG. 3taken along the lines 4--4 of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, wherein like numerals represent likeelements throughout, FIGS. 1-2 show a first preferred embodiment of theopen magnetic resonance imaging (MRI) magnet 110 of the presentinvention. The magnet 110 includes a first superconductive coil assembly112 with a generally toroidal-shaped first coil housing 114 whichsurrounds a first bore 116 and which has a generally longitudinal firstaxis 118. The magnet 110 also includes a second superconductive coilassembly 120 with a generally toroidal-shaped second coil housing 122which surrounds a second bore 124 and which has a generally longitudinalsecond axis 126. The second coil housing 122 is longitudinally spacedapart from the first coil housing 114 by structural posts 128 to createa cylindrical-shaped open space 129 therebetween, and the second axis126 is generally coaxially aligned with the first axis 118. It ispreferred that the coil housings 114 and 122 alone or together (througha hollow structural post 128) define a vacuum enclosure. Preferably, thesecond superconductive coil assembly 120 is a generally mirror image ofthe first superconductive coil assembly 112 about a plane 130 (seen onedge as a dashed line in FIG. 2) oriented perpendicular to the firstaxis 118 and disposed longitudinally midway between the first and secondcoil housings 114 and 122.

The first coil housing 114 includes a first generally-circumferentialoutside surface 132 facing generally towards the first axis 118 and asecond generally-circumferential outside surface 134 radially spacedapart from the first circumferential outside surface 132 and facinggenerally away from the first axis 118. The first coil housing 114 alsoincludes a first generally-annular outside surface 136 facing generallytowards the plane 130 and a second generally-annular outside surface 138longitudinally spaced apart from the first annular outside surface 136and facing generally away from the plane 130.

The first superconductive coil assembly 112 also includes a generallyannular-shaped first superconductive main coil 140 and preferablyincludes generally annular-shaped additional superconductive main coils(not shown in the figures). The additional superconductive main coilsmay be needed to achieve a high magnetic field strength, within themagnet's imaging volume, without exceeding the critical current densityof the superconductor being used in the coils, as is known to thoseskilled in the art. The first superconductive main coil 140 isconventionally supported on a coil form (not shown in the figures). Thefirst superconductive main coil 140 is a DC (direct current) coil havinga generally time-constant operating voltage. The first superconductivemain coil 140 is generally coaxially aligned with the first axis 118, isdisposed within the first coil housing 114, and carries a first mainelectric current in a first direction. The first direction is defined tobe either a clockwise or a counterclockwise circumferential directionabout the first axis 118 with any slight longitudinal component ofcurrent direction being ignored. Hence, the first superconductive maincoil 140 has a first magnetic field direction within the first bore 116which is generally parallel to the first axis 118. The firstsuperconductive main coil 140 typically would be a superconductive wireor superconductive tape wound such that the first superconductive maincoil 140 has a longitudinal extension and a radial extension (i.e.,radial thickness) far greater than the corresponding dimensions of thesuperconductive wire or superconductive tape.

The magnet 110 additionally includes a generally annular-shaped firstresistive coil 144 which is a DC (direct current) coil having agenerally time-constant operating voltage. The first resistive coil 144is generally coaxially aligned with the first axis 118, is spacedradially inward and radially apart from the first superconductive maincoil 140, and carries an electric current in a direction opposite to thefirst direction. Hence, the first resistive coil 144 produces agenerally time-constant magnetic field and has a magnetic fielddirection within the first bore 116 which is generally opposite to thefirst magnetic field direction. The ampere-turns of the first resistivecoil 144 is less than the ampere-turns of the first superconductive maincoil 140. In the first preferred embodiment, as shown in FIG. 2, thefirst resistive coil 144 is disposed within the first coil housing 114proximate the first circumferential outside surface 132 and proximatethe first annular outside surface 136. In an exemplary construction, thefirst resistive coil 144 is a wound copper coil having insulated turns.Preferably, the ampere-turns of the first resistive coil 144 is at leastequal to generally 5% of the ampere-turns of all of the superconductivemain coils in the first coil housing 114.

As previously mentioned and as shown in FIGS. 1 and 2, the secondsuperconductive coil assembly 120 is a generally mirror image of thefirst superconductive coil assembly 112 about the plane 130. Therefore,in addition to the second coil housing 122, the second superconductivecoil assembly 120 also includes a generally annular-shaped secondsuperconductive main coil 146 and preferably includes generallyannular-shaped additional superconductive main coils (not shown in thefigures). It is noted that the additional superconductive main coilswould be needed by the second superconductive coil assembly 120 tobalance any extra additional superconductive main coils of the firstsuperconductive coil assembly 112, as can be appreciated by thoseskilled in the art. The second superconductive main coil 146 isconventionally supported on a coil form (not shown in the figures).

The second superconductive main coil 146 is a DC (direct current) coilhaving a generally time-constant operating voltage and is generallyidentical to the first superconductive main coil 140. The secondsuperconductive main coil 146 is generally coaxially aligned with thesecond axis 126, is disposed within the second coil housing 122, andcarries a second main electric current in the first direction (i.e., inthe same direction as the electric current in the first superconductivemain coil 140). Hence, the second superconductive main coil 146 has asecond magnetic field direction within the second bore 124 which isgenerally identical to the first magnetic field direction.

The magnet 110 further includes a generally annular-shaped secondresistive coil 158 which is a DC (direct current) coil having agenerally time-constant operating voltage. The second resistive coil 158is generally coaxially aligned with the second axis 126, is spacedradially inward and radially apart from the second superconductive maincoil 146, and carries an electric current in a direction opposite to thefirst direction, wherein the first and second resistive coils 144 and158 together correct within an imaging volume 160 for magnetic fieldinhomogeneities caused by (and preferably caused solely by) the presenceof the open space 129, the imaging volume 160 being located in the openspace 129 proximate the first and second axes 118 and 126. Hence, thesecond resistive coil 158 produces a generally time-constant magneticfield and has a magnetic field direction within the second bore 124which is generally opposite to the second magnetic field direction. Theampere-turns of the second resistive coil 158 is less than theampere-turns of the second superconductive main coil 146. In the firstpreferred embodiment, as shown in FIG. 2, the second resistive coil 158is disposed within the second coil housing 122 proximate thecircumferential outside surface which faces radially inward toward thesecond axis 126 and proximate the annular outside surface which faceslongitudinally inward toward the plane 130. In an exemplaryconstruction, the second resistive coil 158 is a wound copper coilhaving insulated turns. Preferably, the second resistive coil 158 is agenerally mirror image of the first resistive coil 144 about the plane130. In an exemplary embodiment, the second resistive coil 158 isgenerally identical to the first resistive coil 144. Hence, theelectrical parameters of the first and second resistive coils 144 and158, such as voltage, current, ampere-turns, etc. are generallyidentical, and the first and second resistive coils 144 and 158 producegenerally identical magnetic fields.

It is noted that the magnet 110 of the invention may be described inother terms as including the previously-described first and secondsuperconductive coil assemblies 112 and 120, the previously-describedimaging volume 160, and the previously-described magnetic fieldinhomogeneities together with presently-described means for correctingthe magnetic field inhomogeneities, such means including thepreviously-described first and second resistive coils 144 and 158. In anexemplary embodiment, such means consists essentially of (and preferablyconsists of) the previously-described first and second resistive coils144 and 158.

The superconductive main coils 140 and 146 of the magnet 110 typicallyproduce a generally spherical imaging volume 160 (shown as a dottedcircle in FIG. 2) centered generally at the intersection of the plane130 and the first axis 118. The effect of the open space 129 between thetwo superconductive coil assemblies 112 and 120 is to distort theuniformity of the magnetic field of the spherical imaging volume 160. Asone moves longitudinally through the spherical imaging volume 160, themagnitude of the magnetic field decreases with decreasing distance fromthe center of the spherical imaging volume 160 because of the missingsuperconductive main coils which were removed to create the open space129. The effect of the resistive coils 144 and 158 is to lower themagnitude of the magnetic field toward the longitudinal edges of thespherical imaging volume 160 in line with the lower magnitude at thecenter. The resistive coils 144 and 158 are designed, using theprinciples of the present invention, previously disclosed herein,together with conventional magnetic field analysis, as is within theskill of the artisan, to produce a highly homogeneous magnetic fieldwithin the spherical imaging volume 160 for improved MRI imaging. It isnoted that the magnet 110 is supported on a conventional magnet floormount 164.

Referring again to the drawings, FIGS. 3-4 show a second preferredembodiment of the open magnetic resonance imaging (MRI) magnet 210 ofthe present invention. Magnet 210 is identical to magnet 110 of thefirst preferred embodiment of the invention, with differences ashereinafter noted. The first resistive coil 244 is disposed outside (andnot within) the first coil housing 214 in the first bore 216, and thesecond resistive coil 258 is disposed outside (and not within) thesecond coil housing 222 in the second bore 224. In a preferredconstruction, the first resistive coil 244 is the only resistive coil inthe first bore 216 which is a DC (direct current) coil producing agenerally time-constant magnetic field, and the second resistive coil258 is the only resistive coil in the second bore 224 which is a DC(direct current) coil producing a generally time-constant magneticfield. Preferably, as shown in FIG. 3, the first resistive coil 244 isattached to the first coil housing 214, and the second resistive coil258 is attached to the second coil housing 222.

It is noted that magnet cooling mechanisms do not form a part of thepresent invention and have been omitted from the figures. Any cryogeniccooling mechanism, such as, but not limited to, liquid helium (or othercryogenic fluid) cooling and/or cryocooler cooling may be employed incombination with the present invention.

The foregoing description of several preferred embodiments of theinvention has been presented for purposes of illustration. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be defined by the claims appended hereto.

We claim:
 1. An open magnetic resonance imaging magnet comprising:a) afirst superconductive coil assembly including:(1) a generallytoroidal-shaped first coil housing surrounding a first bore and having agenerally longitudinal first axis; and (2) a generally annular-shapedfirst superconductive main coil generally coaxially aligned with saidfirst axis, disposed within said first coil housing, and carrying afirst main electric current in a first direction; b) a generallyannular-shaped first resistive coil generally coaxially aligned withsaid first axis, spaced radially inward and radially apart from saidfirst superconductive main coil, and carrying an electric current in adirection opposite to said first direction; c) a second superconductivecoil assembly including:(1) a generally toroidal-shaped second coilhousing longitudinally spaced apart from said first coil housing tocreate a cylindrical-shaped open space therebetween, surrounding asecond bore, and having a generally longitudinal second axis generallycoaxially aligned with said first axis; and (2) a generallyannular-shaped second superconductive main coil generally coaxiallyaligned with said second axis, disposed within said second coil housing,and carrying a second main electric current in said first direction; andd) a generally annular-shaped second resistive coil generally coaxiallyaligned with said second axis, spaced radially inward and radially apartfrom said second superconductive main coil, and carrying an electriccurrent in said opposite direction, wherein said first and secondresistive coils together correct within an imaging volume for magneticfield inhomogeneities caused by the presence of said open space, saidimaging volume located in said open space proximate said first andsecond axes.
 2. The magnet of claim 1, wherein said secondsuperconductive coil assembly is a generally mirror image of said firstsuperconductive coil assembly about a plane oriented perpendicular tosaid first axis and disposed longitudinally midway between said firstand second coil housings, and wherein said second resistive coil is agenerally mirror image of said first resistive coil about said plane. 3.The magnet of claim 2, wherein said first resistive coil is disposedwithin said first coil housing and said second resistive coil isdisposed within said second coil housing.
 4. The magnet of claim 3,wherein said first coil housing has a first generally-circumferentialoutside surface facing generally towards said first axis and whereinsaid first resistive coil is disposed proximate said firstcircumferential outside surface.
 5. The magnet of claim 4, wherein saidfirst coil housing has a first generally-annular outside surface facinggenerally towards said plane and wherein said first resistive coil isdisposed proximate said first annular outside surface.
 6. The magnet ofclaim 2, wherein said first resistive coil is disposed outside saidfirst coil housing in said first bore and said second resistive coil isdisposed outside said second coil housing in said second bore.
 7. Themagnet of claim 6, wherein said first and second resistive coils producegenerally time-constant and generally identical magnetic fields, whereinsaid first resistive coil is the only resistive coil in said first boreproducing a time-constant magnetic field, and wherein said secondresistive coil is the only resistive coil in said second bore producinga time-constant magnetic field.
 8. The magnet of claim 6, wherein saidfirst resistive coil is attached to said first coil housing and saidsecond resistive coil is attached to said second coil housing.
 9. Themagnet of claim 6, wherein said first coil housing has a firstgenerally-circumferential outside surface facing generally towards saidfirst axis and wherein said first resistive coil is disposed proximatesaid first circumferential outside surface.
 10. The magnet of claim 9,wherein said first coil housing has a first generally-annular outsidesurface facing generally towards said plane and wherein said firstresistive coil is disposed proximate said first annular outside surface.